Add delete context menu for custom wallpapers and some ui tweaks

chrome/common/extensions/api/wallpaper_private.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace":"wallpaperPrivate",
     "compiler_options": {
       "implemented_in": "chrome/browser/chromeos/extensions/wallpaper_private_api.h"
     },
@@ skipping 71 matching lines @@
             "name": "url"
           },
           {
             "type": "function",
             "name": "callback",
             "parameters": []
           }
         ]
       },
       {
+        "name": "resetWallpaper",
+        "type": "function",
+        "description": "Clears current user's active custom wallpaper and changes to default wallpaper.",
+        "nodoc": "true",
+        "parameters": []
+      },
+      {
         "name": "setCustomWallpaper",
         "type": "function",
         "description": "Sets wallpaper to the image from local file with specified layout and returns thumbnail if needed.",
         "nodoc": "true",
         "parameters": [
           {
             "type": "binary",
             "name": "wallpaper"
           },
           {
@@ skipping 132 matching lines @@
                 "name": "results",
                 "items": {"type": "string"}
               }
             ]
           }
         ]
       }
     ]
   }
 ]